In copying a manuscript, etc., an electrostatic latent image is first formed on a light-sensitive medium or electrostatic medium film, and then is developed by attaching thereonto fine particles which are prepared by dispersing a colorant in a binder resin; such dispersions are called "toners". The toner image thus formed is then transferred to the surface of a support, such as paper, and is fixed thereon, for example, by heating. For the fixation of the toner image, a heat-fixation method is ordinarily used in which the toner image is heated to a temperature at which the toner melts and flows, and is then melt-adhered to the support to provide a final copy, although methods such as solvent-treatment and overcoating-treatment are also known. As this heat-fixation method, the so-called heat roll fixation method, i.e., a method in which a support bearing thereon a toner image is inserted between a pair of rolls consisting of a heat roll and a pressure-application roll where it is heated under pressure to perform the fixation, has been used in view of the speed of copying.
The heat roll fixation method is very effective, in that the thermal efficiency is high, the heat dissipation is low, and rapid fixation is possible, compared with other heat-fixation methods.
This heat roll fixation method, however, is liable to cause the so-called off-set phenomenon, i.e., a phenomenon that part of toner images sticks to the heat roll or pressure-application roll, because the toner image comes into contact with the roll. In order to prevent the foregoing phenomenon, heretofore, the surface of the roll has been coated with a material having excellent releasability, such as a fluorine resin, and furthermore, an off-set preventing liquid, such as silicone oil, has been supplied onto the surface of the roll. This produces the effect of preventing the off-set phenomenon to a certain extent. However, this technique has disadvantages in that the off-set preventing liquid is required, an apparatus to always supply a constant amount of off-set preventing liquid onto the surface of the roll is required, oil-leakage occurs, and in that the off-set preventing liquid produces odor resulting from the heating thereof.
Recently, attempts to perform heat roll fixation without the use of such off-set preventing liquids have been made. For example, Japanese Patent Publication No. 5549/1978 discloses a method to perform the heat roll fixation without the use of an off-set preventing liquid. In addition, methods to perform the heat roll fixation without the use of an off-set preventing liquid by improving toner particles themselves are described in Japanese Patent Application (OPI) Nos. 42354/1974, 65231/1974, 65232/1974, 107743/1974, 11240/1975, 27546/1975, 28840/1975, 81342/1975, 85338/1975, 93646/1975, 93647/1975, 134652/1975, 144446/1975, etc.
These attempts to prevent the off-set phenomenon in the heat roll fixation by improving toner particles themselves without the use of an off-set preventing liquid have failed to completely satisfy the characteristics required for dry toners. That is, the friction charging properties of toner particles are different, the friction charging properties vary with the lapse of time, and the toner particles themselves cause blocking, lowering developing properties.
Recently, a press-fixation method has been used in place of the heat fixation method. In accordance with this press-fixation method, a support bearing thereon transferred toner images is introduced between a pair of press rolls, and the toner images are fixed on the surface of the support principally by means of pressure. Such toners for press-fixation are required to have the following characteristics: (1) They have a plastic deformation ability due to pressure, (2) When the application of pressure is removed, they do not fluidize; that is, the toner image after the fixation is not easily stripped or peeled apart simply by rubbing. (3) They undergo no aggregation during the storage thereof, e.g., in a toner hopper. (4) They do not form a thick toner layer on the surface of a light-sensitive medium.
To the conventional toners for press-fixation have been added wax and other additives so that the plastic deformation can be accomplished by application of pressure. In the case of such toners with wax or the like incorporated thereinto, when a fixed image is rubbed with a finger, it flows, staining a copied material; friction chargeability between toner particles is different, and it also varies with the lapse of time; and such toners cause blocking, thereby reducing developability. Furthermore, when these toners are used in combination with carriers, the toners attach onto the surface of carriers, exerting adverse influences on the friction chargeability and reducing the developability. Thus, completely satisfactory toners for press-fixation have not yet been discovered.